JERUSALEM — Hamas formally announced the end of its unwritten, often-breached truce with Israel on Friday as Palestinian militants in the Gaza Strip fired four rockets into southern Israel.

The Israeli military said two rockets were fired Friday morning and two more after sunset. It said troops guarding Israeli farmers in fields adjoining Gaza also came under sniper fire from across the border. No injuries were reported in any of the incidents.

In a statement posted on its Web site, the Islamic militant group Hamas said Israel had breached agreements by imposing a painful economic blockade on Gaza, staging military strikes into the densely populated coastal strip and continuing to hunt down Hamas operatives in the West Bank.

"Since the enemy did not abide with the conditions ... we hold the enemy the fully responsible for ending the truce and we confirm that the Palestinian resistance factions headed by Hamas will act," the statement said.

There was no immediate Israeli comment about Hamas' announcement that it would not extend the cease-fire past its end Friday. Israel said previously that the six-month-old truce, brokered by Egypt, didn't have an official expiration date and that the government was interested in prolonging "understandings" with Hamas.

In Washington, Secretary of State Condoleezza Rice was critical of Hamas' move.

"I sincerely hope that there will not be a resumption of the violence because that is not going to help the people of Gaza, it is not going to help the Palestinians, it is not going to help the Palestinian cause," she said.

U.N. Secretary-General Ban Ki-moon reiterated his appeal for a truce extension and an immediate halt to rocket attacks on Israel and all acts of violence, U.N. deputy spokeswoman Marie Okabe said in New York.

"A major escalation of violence would have grave consequences for the protection of civilians in Israel and Gaza, the welfare of the Gazan civilian population, and the sustainability of political efforts," Okabe said.

No official contactsHamas, which violently seized control of the Gaza Strip in June 2007, is listed as a terrorist organization by Israel, the United States and much of the international community and Israel does not officially have direct contacts with it.

Though violence and casualties dropped significantly under the cease-fire agreement, the truce has increasingly unraveled since early November, when Israeli soldiers entered Gaza to destroy a tunnel that the army said could have been used in a cross-border raid. In response, Palestinian militants resumed firing rockets at Israel.

On Thursday, Gaza militants fired 11 rockets and six mortar shells toward Israel and Israel staged at least two airstrikes against rocket squads. The day before, at least 20 rockets were fired at Israel, wounding two people and causing property damage, the army and police said.

The lull has been a relief for people on both sides of the border. A poll published Tuesday indicated that 74 percent of Palestinians and 51 percent of Israelis wanted to extend the cease-fire.

Israeli military officials have said the army was on full alert and recently practiced various simulations of a full-blown return to hostilities. Still, the expectation was that the two sides would step back from the brink.

Just this week Israel's point-man on the issue, senior defense official Amos Gilad, was in Cairo, seeking — through Egyptian go-betweens — to negotiate a truce extension.

Tenuous truceThe number of casualties and rocket attacks dropped sharply after the truce took hold.

In the first six months of this year, 338 Palestinians and 16 Israelis were killed in cross-border violence, according to Associated Press figures. Since the truce took effect at midyear, 21 Palestinians, most of them militants, were killed by Israeli fire. No Israelis were killed.

Israel says Gaza militants fired just 199 rockets in the final half of 2008, compared to 1,786 during the first six months.

During the truce, Hamas has been smuggling weapons through tunnels under the Gaza-Egypt border. Hamas militants are also believed to be burrowing tunnels into Israel to carry out attacks.

Even before the truce began fraying, Israel did not allow the free transfer of goods in and out of Gaza. Since cross-border fighting resumed in November, Israel has kept the borders virtually sealed, allowing in only minimal humanitarian aid.

There were protests against the Gaza blockade in several Mideast states Friday.

In Lebanon, thousands of supporters of the militant group Hezbollah swarmed Beirut's southern suburbs, some chanting "Death to America" and "Israel is the enemy of Muslims." In the southern city of Sidon, about 1,000 Hezbollah supporters staged a sit-in at the main square, halting traffic for about three hours.

About 3,000 Palestinian refugees in Syria joined a demonstration at the Sbeineh camp outside the capital, Damascus.

In the Persian Gulf island state of Bahrain, security forces clashed with thousands of protesters who were demanding that Arab governments take action to end the Gaza blockade.

Witnesses said a number of people, including women and children, were wounded by rubber bullets and others overcome by tear gas. An Interior Ministry spokesman, Maj. Mohamad Bin Dina, denied rubber bullets were used and said tear gas was fired when some demonstrators began destroying public property and throwing stones at police.

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